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A New Horizontal Mixing-Length Formulation for Numerical Simulations of Tropical Cyclones

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  • 1 I.M. System Group, Environment Modeling Center, NOAA/National Centers for Environmental Prediction, College Park, Maryland
  • | 2 Environment Modeling Center, NOAA/National Centers for Environmental Prediction, College Park, Maryland
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Abstract

A new physically based horizontal mixing-length formulation is introduced and evaluated in the Hurricane Weather and Research Forecasting (HWRF) Model. Recent studies have shown that the structure and intensity of tropical cyclones (TCs) simulated by numerical models are sensitive to horizontal mixing length in the parameterization of horizontal diffusion. Currently, many numerical models including the operational HWRF Model formulate the horizontal mixing length as a fixed fraction of grid spacing or a constant value, which is not realistic. To improve the representation of the horizontal diffusion process, the new formulation relates the horizontal mixing length to local wind and its horizontal gradients. The resulting horizontal mixing length and diffusivity are much closer to those derived from field measurements. To understand the impact of different mixing-length formulations, we analyze the evolutions of an idealized TC simulated by the HWRF Model with the new formulation and with the current formulation (i.e., constant values) of horizontal mixing length. In two real-case tests, the HWRF Model with the new formulation produces the intensity and track forecasts of Hurricanes Harvey (2017) and Lane (2018) that are much closer to observations. Retrospective runs of hundreds of forecast cycles of multiple hurricanes show that the mean errors in intensity and track simulated by HWRF with the new formulation can be reduced approximately by 10%.

Significance Statement

To improve the representation of horizontal diffusion in numerical models, this study proposes a new formulation for horizontal mixing length, which calculates the mixing length as a function of local winds. In contrast, current operational models simply assume that the horizontal mixing length is a constant value or a fixed fraction of grid size, which is not realistic. The new formulation produces the horizontal mixing length and diffusivity much closer to those derived from observations than the formulation used in current models. Analyses of retrospective runs of hundreds of forecast cycles suggest that the errors in intensity and track simulated by HWRF with the new formulation can be reduced by 10%. Future work should focus on understanding how large-scale fields and tropical cyclone structure respond to horizontal diffusion parameterizations as well as their impacts on the forecasts of track and intensity.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Weiguo Wang, weiguo.wang@noaa.gov

Abstract

A new physically based horizontal mixing-length formulation is introduced and evaluated in the Hurricane Weather and Research Forecasting (HWRF) Model. Recent studies have shown that the structure and intensity of tropical cyclones (TCs) simulated by numerical models are sensitive to horizontal mixing length in the parameterization of horizontal diffusion. Currently, many numerical models including the operational HWRF Model formulate the horizontal mixing length as a fixed fraction of grid spacing or a constant value, which is not realistic. To improve the representation of the horizontal diffusion process, the new formulation relates the horizontal mixing length to local wind and its horizontal gradients. The resulting horizontal mixing length and diffusivity are much closer to those derived from field measurements. To understand the impact of different mixing-length formulations, we analyze the evolutions of an idealized TC simulated by the HWRF Model with the new formulation and with the current formulation (i.e., constant values) of horizontal mixing length. In two real-case tests, the HWRF Model with the new formulation produces the intensity and track forecasts of Hurricanes Harvey (2017) and Lane (2018) that are much closer to observations. Retrospective runs of hundreds of forecast cycles of multiple hurricanes show that the mean errors in intensity and track simulated by HWRF with the new formulation can be reduced approximately by 10%.

Significance Statement

To improve the representation of horizontal diffusion in numerical models, this study proposes a new formulation for horizontal mixing length, which calculates the mixing length as a function of local winds. In contrast, current operational models simply assume that the horizontal mixing length is a constant value or a fixed fraction of grid size, which is not realistic. The new formulation produces the horizontal mixing length and diffusivity much closer to those derived from observations than the formulation used in current models. Analyses of retrospective runs of hundreds of forecast cycles suggest that the errors in intensity and track simulated by HWRF with the new formulation can be reduced by 10%. Future work should focus on understanding how large-scale fields and tropical cyclone structure respond to horizontal diffusion parameterizations as well as their impacts on the forecasts of track and intensity.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Weiguo Wang, weiguo.wang@noaa.gov
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